One way is to make the check-in person more than a dumb data enterer. My iPhone suddenly stopped getting any carrier signal one day. I took it to the Apple store that evening because I needed to go out of town the next day, and I had to have a working phone. I checked in at the front of the store and waited 2.5 hours for my turn with a Genius. The fix was to press the very specific "reset network settings button" (which isn't under Cellular settings, it's under General->Reset). I was relieved it was such a simple fix, but it was ridiculous to wait over 2 hours to press the right button. The check-in person should ask what you've tried and do the most basic triage to avoid this kind of problem congesting the store.